1. Field of the Invention
This invention relates to a color image reading apparatus and, more particularly, it relates to a color image reading apparatus adapted to separate a light beam coming from a color image into a plurality of light beams having respective wavelengths different from each other by means of a diffraction grating and receive the light beams produced by the separation by means of at least three line sensors.
2. Related Background Art
Various color image reading apparatus have been proposed to date for reading digitized color image information from the output signal of a line sensor by forming a color image of an original on the line sensor having light receiving pixels arranged one-dimensionally along the main scanning direction of the apparatus by way of an optical system. Charge coupled devices (CCDs) are typically used for the line sensor.
FIG. 1 of the accompanying drawings is a schematic cross sectional view of a principal portion of a known color image reading apparatus taken along the sub-scanning direction thereof. In FIG. 1, the X-axis is arranged along the main scanning direction while the Y-axis is running along the sub-scanning direction that rectangularly intersects the main scanning direction. Referring to FIG. 1, the light beam of a color image coming from the surface of the original 61 is converged by a focussing lens 62 and separated into the three primary colors of red (R), green (G) and blue (B) by means of a 3P prism. Then, the light beams of the three primary colors are focussed respectively on line sensors 64, 65 and 66. Each of the line sensors has a plurality of detecting elements (light receiving pixels) arranged one-dimensionally along the main scanning direction so that the color image information is read from each of the light beams having different wavelengths as the color image is scanned along the sub-scanning direction by means of a scanning means such as a mirror (not shown).
However, a known color image reading apparatus as illustrated in FIG. 1 requires the use of a 3P prism that is prepared through an elaborate process to make the apparatus complex and costly. Further more, it is accompanied by various other problems including that the light beams produced by separate the incoming light beam by means of a 3P prism and the respective line sensors have to be positionally adjusted independently to make the entire operation of assembling the apparatus and regulating the performance thereof a very cumbersome one.
FIG. 2 is a schematic cross sectional view of a principal portion of another known color image reading apparatus taken along the sub-scanning direction thereof. The components same as those of FIG. 1 are denoted respectively by the same reference symbols and would not be described any further. In the apparatus of FIG. 2, the 3P prism of FIG. 1 is replaced by a pair of color separating beam splitters 74 and 75 provided with a wavelength selecting transmission film and adapted to divide the light beam converged by the focussing lens 62 into three light beams of the three primary colors that are separated from each other. The divided and separated three light beams are then focussed respectively on the three component line sensors of a monolithic 3-line sensor unit 73 arranged on the surface of a same substrate. Then, the color image information is read from the light beams having different wavelengths as the color image is scanned along the sub-scanning direction by means of a scanning means such as a mirror (not shown).
With the color image reading apparatus of FIG. 2, if the beam splitters 74 and 75 have a thickness of x, the distance separating the line sensors will be equal to 22x. Then, if the desired distance separating the line sensors is between 0.064 and 0.2 mm, the beam splitters 74 and 75 should be made to show a thickness x between 23 and 70 xcexcm.
Normally, it is highly difficult to prepare beam splitters that have such a small thickness and still maintain optically excellent planeness. Therefore, it is also highly difficult for an apparatus as shown in FIG. 2 to focus light beams on the respective line sensors without reducing their optical effectiveness.
FIG. 3 is a schematic cross sectional view of a principal portion of still another known color image reading apparatus taken along the sub-scanning direction thereof. The components same as those of FIG. 1 and FIG. 2 are denoted respectively by the same reference symbols and would not be described any further. In the apparatus of FIG. 3, the color image on the surface of the original 61 is read only by means of an objective lens 62 and a monolithic 3-line sensor unit 73 same as that of FIG. 2. FIG. 4 is a schematic perspective view of the monolithic 3-line sensor unit 73.
Referring to FIGS. 3 and 4, the monolithic 3-line sensor unit 73 has three component line sensors 81, 82 and 83 arranged in parallel with each other on a same substrate. Each of the line sensors typically comprises a charge coupled device having a plurality of detecting elements (light receiving elements) arranged one-dimensionally along the main scanning direction. If the line sensors 81 and 82 are separated from each other by a distance of S1 and the line sensors 82 and 83 are separated from each other by a distance of S2, both S1 and S2 are normally made to take a value between 0.064 and 0.2 mm in view of various manufacturing conditions. If the detecting elements (light receiving elements) 84 of the line sensors have a width W1 in the main scanning direction and a width W2 in the sub-scanning direction, they are normally made to take a value between 8 and 10 xcexcm. If W1 and W2 are made equal to each other, a single detecting element may be made to have a size of 8 xcexcmxc3x978 xcexcm or 10 xcexcmxc3x9710 xcexcm. The line sensors 81, 82 and 83 are provided thereon with color filters for transmitting only light beams of blue (B), green (G) and red (R) respectively.
Generally, the gap S1 separating the line sensors 81 and 82 and the gap S2 separating the line sensors 82 and 83 are made equal to each other and also equal to the value of the pixel size W2 along the sub-scanning direction as shown in FIG. 4 multiplied by an integer for the reason as described below. Referring to FIG. 3, if only a focussing lens 62 is used to read the color image by means of a monolithic 3-line sensor unit as described above, the three line sensors 81, 82 and 83 will read the original 61 simultaneously at three different respective positions 81xe2x80x2, 82xe2x80x2 and 83xe2x80x2. Then, it is impossible to read simultaneously the signal components of the three primary colors (R, G, B) at any point on the original 61. This means that the signal components of the three primary colors have to be synthetically combined after reading them respectively by means of the three sensors.
This operation of synthetically combining the signal components of the three primary colors can be carried out advantageously by selecting each of the inter-line distances S1, and S2 of the three line sensors so as to be equal to the pixel size W2 multiplied by an integer and using a corresponding redundancy line memory in order to delay, for example, the G and R signals (signal components for G and R) relative to the B signal (signal component for B). Thus, generally, S1 and S2 are made equal to each other and to W2 multiplied by an integer.
Meanwhile, there have been proposed apparatus comprising a monolithic 3-line sensor unit and adapted to read a color image at a same position for the three primary colors without delaying any of the signal components. FIG. 5 is a schematic cross sectional view of a principal portion of such a known color image reading apparatus taken along the sub-scanning direction thereof. In FIG. 5, the components same as those of FIG. 4 are denoted respectively by the same reference symbols and would not be described any further.
Referring to FIG. 5, the light beam from a color image on the original 61 is converged by a focussing lens 62 and diffracted and color-separated into three light beams having respective wavelengths different from each other by means of a transmission type one-dimensional blazed diffraction grating 101. The three light beams produced by the color separation are then focussed on the respective line sensors of the monolithic 3-line sensor unit 73 so that the color signals for a given position of the original 61 can be read simultaneously. For instance, the color signal of red (R) will be detected by means of light of 0 order and that of green (G) will be detected by means of light of 1 order, while the color signal of blue (B) will be detected with light of xe2x88x921 order.
However, a known color image reading apparatus as shown in FIG. 5 is accompanied by a problem as discussed below. Assume that the one-dimensional blazed diffraction grating 101 performs with a wavelength characteristic as illustrated in FIG. 6. Then, the angle of light of +1 order relative to light of 0 order would not agree with that of light of xe2x88x921 order to give rise to a problem of asymmetry regardless of the selected pitch of the diffraction grating. Then, when a monolithic 3-line sensor unit 73 where three line sensors are arranged at regular intervals is used as described above, the focussed positions of the three light beams are displaced from the respective line sensors. While this problem may be dissolved by preparing a specifically designed 3-line sensor unit comprising three line sensors that are arranged at differentiated intervals for the color image reading apparatus, such a unit is highly costly.
Therefore, it is the object of the present invention to solve the above identified technological problems of the prior art and provide a color image reading apparatus that has a simple configuration and hence can be manufactured at low cost but is adapted to read color images highly accurately.
According to the invention, the above object is achieved by providing a color image reading apparatus comprising:
at least three line sensors arranged at regular intervals along the sub-scanning direction perpendicular to the main scanning direction, each having a plurality of light receiving pixels arranged along the main scanning direction;
an imaging optical system including at least one lens for focussing a light beam coming from a color image located at the reading position of the apparatus on said line sensors; and
a diffraction grating for separating the light beam coming from said color image into a plurality of light beams having respective wavelengths different from each other;
the optical axis of the at least one lens of said imaging optical system being eccentrically disposed so as for said plurality of light beams produced by said diffraction grating to be focussed on the respective line sensors.